1. Field of the Invention
The present invention relates to a speed reducer designed to obtain an output rotation reduced from an input rotation and to be used for speed reduction in a robot, a traveling or revolving device for construction machines or the like, or a windmill.
2. Description of the Related Art
Heretofore, three has been known a differential/wobbling or oscillating type speed reducer, such as a cyclo speed reducer, which is designed to revolve an externally-toothed gear fitted onto an eccentric portion while being meshed with an internally-toothed gear, so as to obtain an output rotation reduced from an input rotation. Typically, in this differential/oscillating type speed reducer, a crankshaft is provided with two eccentric portions with a given phase difference therebetween, and two externally-toothed gears each fitted onto a corresponding one of the eccentric portions is in meshing engagement with the pin teeth. The pin teeth are arranged along an inner peripheral surface of a case at given circumferential intervals, and the phase difference between the two eccentric portions is set at 180 degrees. Thus, in conjunction with a rotation of the crankshaft, the two externally-toothed gears are revolved with the phase difference while being meshed with the pin teeth, so as to obtain an intended output rotation. Each of the pin teeth in meshing engagement with the externally-toothed gears is adapted to be rotated on its axis by a force received from each of the externally-toothed gears during the above movement. This prevents the externally-toothed gears from being slidingly moved relative to the pin teeth, so as to reduce a rotational resistance of the externally-toothed gears. In addition, the externally-toothed gears each designed to be in meshing engagement with the pin teeth in an angular range of 180 degrees make it possible to reduce a load to be imposed on each of the pin teeth.
There has also been known a differential/oscillating type speed reducer which is provided with three eccentric portions and three externally-toothed gears each arranged on a corresponding one of the eccentric portions, as disclosed, for example, in Japanese Patent Laid-Open Publication No. 64-15556. In this speed reducer, the three eccentric portions are arranged to have a phase difference of 120 degrees with respect to each other.
In a differential/oscillating type speed reducer provided with three externally-toothed gears as disclosed in the Japanese Patent Laid-Open Publication No. 64-15556, if each of the three externally-toothed gears is designed to be in meshing engagement with the pin teeth in an angular range of 180 degrees as in the conventional manner, the three externally-toothed gears will be in meshing engagement with the pin teeth in a total angular angle of 540 degrees. This means that there exist certain ones of the pin teeth are simultaneously in meshing engagement with two of the externally-toothed gears. Thus, each of the certain pin teeth simultaneously receives respective forces from the two externally-toothed gears, and these forces are different from each other in direction and magnitude. Consequently, at least one of the two externally-toothed gears will be slidingly moved relative to the certain pin teeth. As above, if a differential/oscillating type speed reducer is designed to have three externally-toothed gears under the condition that an angular range of meshing engagement between each of the externally-toothed gears and the pin teeth is set at 180 degrees without modification, the effect of allowing each of the pin teeth to be rotated on its axis so as to reduce a rotational resistance of the externally-toothed gears, as in the differential/oscillating type speed reducer provided with two externally-toothed gears, cannot be obtained to cause a problem about an increase in rotational loss of the differential/oscillating type speed reducer.
In the conventional differential/oscillating type speed reducer disclosed in the Japanese Patent Laid-Open Publication No. 64-15556, each of the three externally-toothed gears is fitted onto a corresponding one of the three eccentric portions of the eccentric shaft through a bearing comprising a retainer adapted to hold a plurality of rollers (rolling elements). Further, each of the three externally-toothed gears is in meshing engagement with an internally-toothed gear. Thus, when a certain torque is given from an input shaft to the externally-toothed gears through the respective eccentric portions of the eccentric shaft, the externally-toothed gears will be revolved while being meshed with the internally-toothed gear.
In this conventional differential/oscillating type speed reducer, the three eccentric portions have a phase difference with respect to each other. Thus, when one of the bearings is attached from the side of one end of the eccentric shaft along an axial direction of the eccentric shaft and then fitted onto an intermediate one of the eccentric portions, the rollers of the bearing are liable to interfere with the eccentric portion on the side of the end, i.e., the first eccentric portion. In order to avoid this interference, the retainer of the bearing is designed to have a wobbling movement for allowing the rollers to be displaced radially outward. Specifically, before the bearing is moved to pass through the first eccentric portion, the rollers of the bearing are displaced radially outward. Then, the bearing in this state is moved to pass through the first eccentric portion and fitted onto the intermediate eccentric portion. This allows the bearing to be moved along the eccentric shaft and fitted onto the intermediate eccentric portion having a phase difference relative to the remaining eccentric portions.
With a view to downsizing a differential/oscillating type speed reducer, a diameter of each of the externally-toothed gears is reduced to allow the differential/oscillating type speed reducer to have a smaller diameter, in some cases. In these cases, a load to be imposed from the eccentric portions of the eccentric shaft onto the rollers of the bearings is increased to cause a disadvantage of deterioration in durability of the rollers. As measures for solving this disadvantage, there has been known a so-called full-type roller bearing. This bearing is disclosed, for example, in Japanese Patent Laid-Open Publication No. 2005-265126.
In the bearing disclosed in the Japanese Patent Laid-Open Publication No. 2005-265126, only a plurality of rollers are disposed along an outer peripheral surface of an eccentric portion without interposing any other member therebetween. This makes it possible to increase the number of rollers to be disposed around the eccentric portion, so as to further disperse a load to be imposed from the eccentric portion onto each of the rollers to solve the above disadvantage. Further, the bearing disclosed in the Japanese Patent Laid-Open Publication No. 2005-265126 is provided with a plurality of presser arms for pressing each of the plurality of rollers disposed around the eccentric portion, inward from an outward side of the rollers. That is, the rollers are held by the presser arms so as not to fall out of the eccentric portion.
However, in the conventional differential/oscillating type speed reducer disclosed in the Japanese Patent Laid-Open Publication No. 64-15556, the retainer of the bearing to be fitted onto the intermediate eccentric portion is designed to have a wobbling movement, as mentioned above. Thus, even after the bearing is fitted onto the intermediate eccentric portion, the rollers of the bearing will wobble to cause a disadvantage of difficulty in arranging the rollers around the intermediate eccentric portion at even intervals.
As measures for solving this disadvantage, it is contemplated that only an intermediate one of the three eccentric portions is formed to have a larger diameter so as to absorb wobbling movements of the rollers to suppress wobbling movements of the rollers after the fitting of the bearing. However, in this structure, a mounting hole of the externally-toothed gear to be fitted onto the intermediate eccentric portion is required to be increased in diameter in conformity to a diameter of the intermediate eccentric portion. Consequently, among the three eccentric portions, only the intermediate eccentric portion will have a different configuration to cause problems about an increase in number of component types and complexities in production process and component management during production.
The full-type roller bearing disclosed in the Japanese Patent Laid-Open Publication No. 2005-265126 has a disadvantage in that there is the difficulty in suppressing inclination of the rollers when a certain force is applied from the eccentric portion to the rollers. Specifically, in the bearing disclosed in the Japanese Patent Laid-Open Publication No. 2005-265126, each of the presser arms is deformable in response to a certain force applied thereto, because one end of the presser arm is not fixed. Thus, when a certain force is applied from the eccentric portion to the rollers, the presser arms are displaced from their proper positions due to the force to cause a disadvantageous phenomenon that each of the rollers wobbles and inclines in an oblique direction. If the roller inclines in an oblique direction, an excessive load is likely to be imposed from the eccentric portion onto the roller to cause a problem about breakage of the roller.
Heretofore, there has been a camshaft integrally having a plurality of cams. This camshaft is disclosed, for example, in Japanese Patent Laid-Open Publication No. 2004-36662.
In the camshaft disclosed in the Japanese Patent Laid-Open Publication No. 2004-36662, a camshaft material is subjected to a polishing process using a grindstone or the like, to form each portion of the camshaft into a given shape. In a camshaft designed such that first and second adjacent cams are disposed closer to each other, when an edge of the first cam is formed, an accurate polishing operation is likely to be hindered by interference between the grindstone and the second cam. Thus, the above conventional camshaft is designed such that first and second adjacent cams are disposed spaced apart from each other by a given distance, and connected to each other by a shaft portion formed in a given area less than an overlapping area between the first and second cams when viewed in an axial direction of the camshaft. More specifically, in this camshaft, the first and second adjacent cams are connected to each other through the shaft portion in such a manner as to avoid interference between a grindstone and a camshaft material even if the grindstone protrudes toward the second cam during an operation of polishing an edge of the first cam. Thus, the cams including edges thereof can be subjected to a polishing process with a high degree of accuracy.
The structure of the camshaft disclosed in the Japanese Patent Laid-Open Publication No. 2004-36662 may be applied to the crankshaft integrally formed with the plurality of eccentric portions each having a different rotational phase. In this case, a connection portion for connecting the adjacent eccentric portions to each other is formed in a given area less than an overlapping area between the adjacent eccentric portions when viewed in an axial direction of the crankshaft. However, in this structure, a sectional area of the connection portion becomes smaller, and therefore the strength of the connection portion will be lowered. This causes a problem about deterioration in strength of the crankshaft.
Heretofore, there has also been known a differential/oscillating type speed reducer designed to revolve an externally-toothed gear member while being meshed with an internally-toothed gear member, through an eccentric portion, so as to obtain an output rotation reduced or increased from an input rotation. This differential/oscillating type speed reducer is disclosed, for example, in Japanese Patent Laid-Open Publication No. 2003-83400. As shown in FIGS. 34 and 35, the differential/oscillating type speed reducer disclosed in the Japanese Patent Laid-Open Publication No. 2003-83400 comprises a cylindrical-shaped outer case (internally-toothed gear member) 381 provided with internal teeth 381a along an inner periphery thereof, a carrier 382 disposed coaxially with the outer case 381 in a rotatable manner relative to the outer case 381, and a pinion (externally-toothed gear member) 383 in meshing engagement with the internal teeth 381a of the outer case 381. The carrier 382 serves as an output shaft, and includes a base member 382a, a sectionally approximately-triangular-shaped column portion 382b provided on the base member 382a, and an end plate 382c fastened to the column portion 382b. Two of the pinions 383 are disposed in an axial direction of the outer case 381. Each of the pinions 383 is penetrated by the column portion 382b and a crankshaft 384 provided with an eccentric portion 384a. Four of the crankshafts 384 and four of the column portions 382b are arranged in a circumferential direction of the outer case 381. Each of the crankshafts 384 is rotatably supported by a pair of upper and lower crankshaft bearings 385, 386 disposed, respectively, at upper and lower ends thereof. The upper crankshaft bearings 385 are provided on the end plate 382c, and the lower crankshaft bearings 386 are provided on the base member 382a. Each of the crankshafts 384 is adapted to be rotated in conjunction with an input shaft 388 through a gear 387. When the crankshafts 384 are rotated in conjunction with a rotation of the input shaft 388, each of the pinions 383 is revolved while being meshed with the internal teeth 381a of the outer case 381, according to a rotation of a corresponding one of the eccentric portions 384a. Then, according to the revolutions of the pinions 383, the column portions 382b are revolved to rotate the carrier 382.
An attempt to reduce an outer diameter of the above differential/oscillating type speed reducer involves the need for arranging each of the crankshafts 384 at a position closer to the axis of the outer case 381 and reducing each diameter of the pinions 383. In this case, when each of the crankshafts 384 is arranged at a position closer to the axis of the outer case 381, a load acting on each of the crankshafts 384 will be increased. Further, when each diameter of the pinions 383 is reduced, a load acting on each of the pinions 383 will be increased, and therefore a load acting on each of the crankshafts 384 through the pinions 383 will be increased. Thus, an attempt to reduce an outer diameter of the differential/oscillating type speed reducer while maintaining an output torque involves the need for increasing a supporting rigidity for each of the crankshafts 384. To this end, the crankshaft bearings 385, 386 are inevitably increased in size to place limits on reduction in outer diameter of the differential/oscillating type speed reducer.